The relatively new trend in silicon carbide formation is to make it by thermal plasma processes. The high temperatures available in the plasma increase the reaction kinetics by several orders of magnitude and fast quenching rates produce very small particles at high conversion rates, thus providing a number of advantages over older methods for producing very fine, submicron powders of SiC. The gas phase synthesis conducted in a pure and controlled atmosphere at a high temperature gives the powder which is produced properties which are very desirable in subsequent fabrication. These properties include high sphericity, a small diameter and a narrow size distribution.
Plasma processing has a wide range of potential applications, ranging from coating of thin layers on substrates to the destruction of toxic wastes. One of the many promising areas of plasma processing is the production of ultra-time (submicron size) powders of high-value materials (such as carbides and nitrides). Powders produced in a pure and controlled atmosphere may be essential for subsequent fabrication of advanced materials. Silicon carbide has many eminent properties, such as: high refractoriness, high oxidation resistance and high hardness. It also has a thermal conductivity comparable to the metals, and its thermal expansion coefficient is relatively low compared with other ceramics. Because of these properties, silicon carbide can be effectively used for high temperature mechanical applications. The products obtained by the present invention can be employed for those purposes for which ceramic carbides are presently used.
A fundamental prerequisite for producing such structural ceramics depends on the availability of relatively inexpensive, high purity, reproducibly-sinterable SiC powders. One of the more important problems in the application of SiC is its poor sinterability, which is due to strong covalent bonds between molecules. In order to enhance sintering characteristics, the silicon carbide powder must have a uniform particle size distribution and a submicron mean particle size.
There have been a number of investigations concerning the production of SiC powder in plasma reactors. In most of the investigations, the reactants used (e.g., silane) are quite expensive. These reactants are used because they are easily vaporized and therefore easily converted. Silicon carbide has been produced using inexpensive reactants, such as silica and hydrocarbons, with an RF plasma. However, RF plasmas may present a thermal efficiency problem when scaled up to an industrial size.
Many investigators have studied and analyzed the mechanism and kinetics of silicon carbide formation in the silica-carbon system. They have proposed different intermediate species during the reaction, which vary over the temperature range used. The overall reaction between silica and carbon, which is endothermic, may be written as: EQU SiO.sub.2 (s)+3C (s)=SiC (s)+2CO (g)
This reaction, as written, is very slow even under plasma reactor conditions; so the reaction rates must be increased by the formation of gaseous intermediaries. In this invention, the reactants for the SiC formation are silica and methane. When silica is exposed to high temperature (&gt;2839.degree. C.), it disassociates into silicon monoxide and oxygen, i.e., EQU SiO.sub.2 (s)=SiO(g)+O(g)
H. L. Schick, "A Thermodynamic Analysis of the High-Temperature Vaporization Properties Silica," Chem. rev. (1960), gave a detailed thermodynamic analysis of the high temperature vaporization properties of silica. D. M. Caldwell, "A Thermodynamic Analysis of the Reduction of Silicon Oxides Using a Plasma," High Temperature Science, (1976), presented a computer model for the silicon-oxygen-carbon system. He showed that a threshold temperature exists, at approximately 2400.degree. K., for the maximum yield of silicon and silicon carbide.
When methane is exposed to high temperature, it decomposes into different species depending upon the temperature. The important reactions for the methane decomposition are as follows: EQU 2CH.sub.4 (g)=C.sub.2 H.sub.2 (g)+3H.sub.2 (g) EQU 2CH.sub.4 (g)=2C (s)+4H=(g) EQU 2CH.sub.4 (g)=C.sub.2 H (g)+3.5H.sub.2 (g)
The formation of acetylene by the thermal decomposition of methane is explained by the theory of free radicals and has been observed by a number of investigators, including the work reported here. The primary species formed when methane is exposed to high temperatures are: H.sub.2, C.sub.2 H.sub.2, C.sub.2 H and C.